This invention relates to the field of high-pressure arc discharge lamps and is especially applicable to such lamps having a metallic halide fill. More particularly, the invention relates to lamps containing high-pressure arc tubes made of fused silica having main electrodes at both ends and a starting electrode, or probe, at one end and having the problem of electrolysis inherent to the starter electrode configuration.
High-pressure metal halide arc discharge lamps generally comprise an elongated arc tube containing an ionizable fill and having press seals at each end of the tube. Disposed within the arc tube are two main electrodes, one at each end. The electrodes are generally supported in the press seals and are usually connected to a thin molybdenum ribbon, disposed within the press seal, the purpose of the ribbon being to prevent seal failures because of thermal expansion of the lead-in wire.
In order to facilitate starting of the arc discharge, that is, ionizing of the gas fill, a starting electrode is generally disposed within the arc tube, adjacent one of the main electrodes. Such an electrode is used because an arc can be ignited between the starter electrode and its adjacent electrode at a much lower starting voltage than is required to ignite an arc between the two main electrodes. That is, placement of the starting electrode in the vicinity of one of the main electrodes reduces the gap size of the starting arc, and thereby reduces the ignition voltage requirement. Once the arc has ignited, the ionizing gas decreases the resistance between the two main electrodes and an arc is formed therebetween.
During operation of some metal halide lamps containing alkali or alkaline earth additives, electrolysis between the starting electrode and the adjacent main electrode can occur at the press seal, if there is an electric potential therebetween. The electrolysis current consists mainly of alkali ion flow and, thus, is greater in an arc tube having a fill that includes an alkali than in one that does not. However, electrolysis can always be present since the arc tube material, generally fused silica (high silica glass or quartz), usually contains minute quantities of alkali metals. Hence, because of the alkali in the fused silica or alkaline earth additives in the case of metal halide lamps, migration of the alkali ions occurs between starter and main electrode if an asymmetric potential difference occurs between those two electrodes.
At the necessarily high operating temperatures of the fused silica arc tubes, reactions between the alkali ions and fused silica and molybdenum result causing a difference in the coefficient of expansion between the newly contaminated area and the surrounding fused silica. The stress produced by the differences in expansion coefficient can cause fracture within a short period of time.
Electrolysis occurs only when the starter electrode is negative with respect to the adjacent electrode. Thus, even when the lamp is energized by an AC voltage, the starting electrode can be negative with respect to the adjacent electrode 50% of the time, unless suitable means are employed to prevent a potential thereacross.
Several prior art approaches have been employed to limit the aforementioned electrolysis problem. One such method is described in U.S. Pat. No. 3,226,597 Green, wherein a bimetal switch is used to short out the starting electrode to the adjacent main electrode when the lamp reaches normal operating temperatures. A short period of time, say, about 30 seconds was all that was normally required for the switch to heat up sufficiently and deflect and short the wires. As long as the wires were shorted, and, thus, were at the same potential, no electrolysis could occur between the two electrodes. However, electrolysis could occur during the period of time required for the switch to close.
During operation of the lamp, prolonged exposure of the switch to the heat emanating from the arc tube could cause the bimetal to take a "set" in the stressed position, with the result that the switch could require progressively longer time intervals to close. In some cases, the physical characteristics of the bimetal could be altered sufficiently to prevent closing of the switch altogether or to cause the switch to remain closed even at room temperature. In the latter case, the lamp could not normally be restarted.
Another method of solving the electrolysis problem is described in U.S. Pat. No. 3,619,711 Freese, wherein a semi-conductor diode is located such that the starter electrode cannot develop a negative potential. In many applications, semi-conductor devices have proved to be a very acceptable electrolysis preventative. However, in certain hot fixture applications, premature failure of the diodes can cause starting problems in ensuing cold weather starting situations. Semi-conductor devices are also comparatively costly.
Yet another approach is described in U.S. Pat. No. 3,906,275 Spiessens et al, wherein a metal screening element is sealed in the pinch between the lead-in conductors and does not intersect the internal surface of the envelope.
More recently, U.S. Pat. No. 3,909,660 Sulcs et al describes the use of a very short starting electrode, with the distance from the adjacent main electrode maximized within press seal constraints to essentially eliminate conduction between the starting electrode and the main electrode. The short probe is effective in most lamp types but is comparatively unreliable in metal halide lamps having improved convection currents, such as described in U.S. Pat. Nos. 3,883,766 and 3,896,326 of Fohl. The improved circulation of hot gases causes sufficient heating of the stub electrode to cause significant conduction on one-half cycle, thereby causing electrolysis and failure in a short period of time. Also, in manufacturing, it is difficult to maintain a minimal projection into the quartz arc tube without completely burying the probe and thereby eliminating the usefulnes of the starter electrode.